Conventionally, during recording, disk drive devices write data onto an optical disk based on a reference clock (i.e. write clock) which has a predetermined frequency. Disk drive devices are configured so as to generate, during reproduction, using a PLL (Phase Locked Loop), a read clock corresponding to a reproduction RF signal read from the optical disk, and demodulate data by performing signal processing based on the read clock. (For instance, refer to Patent Document 1).
Therefore, in order to have a disk drive device correctly demodulate data, it is required that the frequency of the read clock match the frequency of the write clock. However, a reproduction RF signal may be corrupted due to various causes such as failures or defects of the optical disk, or impact caused to the disk drive device and the like, which make it impossible to perform accurate PLL on the reproduction RF signal, which in turn may cause fluctuations of the frequency of the read clock. In this case, the frequency of the read clock deviates from the frequency of the write clock, and data cannot be normally reproduced.
Thus, disk drive devices are configured so that the frequency difference between a write clock and a read clock is monitored upon reproduction, and when the frequency difference exceeds a predetermined threshold, it is determined that the frequency of the read clock is inappropriate due to some error, and normalization of data reproduction is attempted by executing a retry operation or changing operation modes.
One of the methods of such monitoring of the frequency of the read clock involves monitoring the state of consistency of a pulse based on an edge of an N-divider signal of the write clock and a pulse based on an edge of an N-divider signal of the read clock, and when discrepancies between the two pulses continue for a predetermined number of times, the frequency of the read clock is deemed to be inappropriate and a transition is made to a read clock NG status, (hereinafter simply referred to as a “NG status”), and when matches between the two pulses continue for a predetermined number of times during a NG status, the frequency of the read clock is deemed to have returned to an appropriate condition, and the status is returned to a read clock OK status (hereinafter simply referred to as an “OK status”).
Another method of monitoring the frequency of the read clock involves monitoring whether an edge of an N-divider signal of the write clock is within a detection window based on an edge of an N-divider signal of the write clock, and when a condition where the edge is not within the detection window continues for a predetermined number of times a transition is made to a NG status, and when a condition where the edge is within the detection window continues for a predetermined number of times during the NG status, the status is returned to an OK status.
[Patent Document 1] Japanese Patent Laid-Open No. 3-201268.
However, in actuality, there are cases when data is normally processed by circuits in subsequent stages even when the frequency of a read clock deviates and a NG status is determined. For instance, when a temporary deviation of the frequency of a read clock occurs due to causes such as a defective portion of an optical disk being read, a NG status may be determined even though a decoder circuit continues to stably detect a frame sync and there is a possibility that a normal decoding will be performed (excessive NG determination), thereby giving rise to a problem wherein a meaningless retry operation is commenced.
In addition, there was a problem at the disk drive device where, in the event that a reproduction RF signal is corrupted due to causes such as a unrecorded portion of an optical disk being read, in turn causing a PLL to become extremely unstable and the read clock to become temporarily extremely quickened, an edge of the read clock enters each detection window despite the fact that there is a major discrepancy between the frequencies of the read clock and the write clock, and an erroneous determination of an OK status is temporarily made, in turn causing the status to fluctuate between OK statuses and NG statuses within a short period of time (destabilization of status determination), and a stable reproduction operation becomes impossible.